1. Field of the Invention
The present invention relates to a dust sampling device, more particularly, to a dust sampling device having a sampling nozzle with a variable nozzle-inlet diameter, wherein a dust sample indispensable for measuring physical characteristics, such as, for example, a concentration, a particle size and a chemical component of particulate substances (hereinafter referred to as xe2x80x9cdustxe2x80x9d) contained in a flue gas discharged from a factory, an automotive vehicle and the like is sampled into the dust sampling device under a predetermined sampling flow rate at the same velocity as a flow velocity of the flue gas. The sampling at the same velocity as the flow velocity of the flue gas is referred to as xe2x80x9cisokinetic samplingxe2x80x9d hereinafter.
2. Description of the Related Art
Recently, much attention has been devoted to the particle size of the dust contained in the flue gas to determine physical properties of the flue gas With the increased demand of the dust measurement in various sources, a problem associated with a conventional measuring method it, for example, a multi-cyclone utilizing a centrifugal force or a measuring method such as Cascade Impactor or Andersen stack sampler utilizing inertia force of particles has been pointed out. The problem of the conventional measuring methods is that the sampling flow rate within the measuring device should be kept constant throughout the operation in case where the dust is sorted according to the particle diameter. Namely, when the dust sample is to be collected from the flue gas at the isokinetic sampling flow rate, it is necessary to measure the flow velocity, the temperature, the pressure, the moisture content and the like of the flue gas in advance, to obtain the isokinetic sampling flow rate in accordance with the following formula based upon a diameter of the sampling nozzle inlet to be used and to perform the sampling and sorting operation of the dust sample for a predetermined period of time.   qm  =            π      4        ⁢          d      2        ⁢          v      ⁢              (                  1          -                                    X              w                        100                          )              ⁢                  273        +        θm                    273        +        θs              xc3x97                            P          ⁢                      xe2x80x83                    ⁢          a                +        Ps                              P          ⁢                      xe2x80x83                    ⁢          a                +        Pm              xc3x97    0.06  
where
gm is the isokinetic sampling flow rate (l/min)
d is the diameter of the nozzle inlet (mm)
v is the flow velocity of the flue gas (m/s)
Xw is the moisture content (%)
xcex8m is the temperature of the sampled gas at a dry gas meter (xc2x0 C.)
xcex8s is the temperature of the flue gas (xc2x0 C.)
Pa is the atmospheric pressure (mmHg)
Ps is the static pressure of the flue gas (mmHg)
Pm is the gauge pressure in a dry gas meter of the sampled gas (mmHg)
Accordingly, in case where the flue gas varies, it is necessary to re-calculate the isokinetic sampling flow rate, which creates problems in measurement operation and requires a great amount of labor and time resulting from the interruption of the measurement, and the exchange of the sampling nozzle.
In the dust measurement in the flue gas (concentration, particle size distribution, chemical composition and the like), it is indispensable for the sampling work to perform the sampling of the flue gas sample through the sampling nozzle at the same flow velocity as that of the flue gas contrary to the gas measurement in the flue gas. If the isokinetic sampling is deviated from a predetermined allowable range (xe2x88x925% to +10% of the flue gas flow velocity) due to the inertia effect resulting from the particle size, flow rate and density of the dust, there is caused a large error in entrainment of the dust particles into the sampling nozzle. For this reason, the flow velocity, temperature, pressure, and moisture content of the flue gas are measured in advance so that the diameter of the sampling nozzle inlet to be used is selected, to thereby calculate the isokinetic sampling flow rate. Thereafter, the sampling of the dust is performed (this method is generally referred to as xe2x80x9cnormal sampling methodxe2x80x9d).
In contrast, FIG. 3 shows an equilibrium sampling method (see Japanese Utility Model Examined Publication No. Sho 56-2191). In this method, a dynamic pressure corresponding to a flow velocity of the flue gas at the sampling point within a duct 21 is displayed at a slant water head meter 38 through pressure feed pipes 37 and 40. A pump 27 is operated to sample (suck) the flue gas within the duct 21 from a sampling nozzle 24. In this manner, an average pressure difference in gas between the upstream and downstream sides of the throttle of a venturi pipe 25 is displayed at a slant water head meter 33. A bypass cock 29 is adjusted so that the average pressure difference indicated in the slant water head meter 33 is identified with the dynamic pressure indicated in the slant water head meter 38, whereby the flue gas is sampled from the sampling nozzle 24 at the same flow velocity as the flow velocity of the flue gas within the duct 21 and the dust is collected into a dust collector 23. An amount of sample flue gas is accumulated and calculated by a gas meter 29. A mass concentration of the dust contained in the flue gas is obtained immediately from the amount of the flue gas sampled and the mass of the dust. This equilibrium type sampling method is useful, because the isokinetic sampling is immediately performed only by identifying the dynamic pressure or the static pressure of the sample gas within the nozzle with the dynamic pressure or the static pressure of the flue gas.
In the conventional measuring method described above, the sampling nozzle is fixed and the isokinetic sampling is performed by changing the sampling flow rate.
In the measuring device in which the sampling flow rate within the measuring device must be kept constant, the sampling flow rate within the measuring device is changed, if the flue gas flow velocity is changed which results in changes in the gas flow rate to be sampled at a velocity corresponding to the flow velocity of the flue gas. This creates a problem in measurement.
In order to overcome such a problem, a flue gas circulation system as shown in FIG. 4 is currently proposed. In this system, in order to keep the sampling flow rate within the measuring device constant, a part of the sample gas is circulated and introduced forward in the measuring device and merged and circulated Smith the gas sampled at an uniform velocity from the nozzle inlet. However, this system is not practical in operation, because it is required to perform cleaning of the circulated flue gas, complicated and enlarged the circulation system, additional equipment such as circulation pumps, and so on. Accordingly, this system is not suitable for the practical use in the on-the-spot measurement in a factory.
According to the present invention, there is provided a measuring device in which the sampling flow rate within the device is kept constant in principle during the dust measurement in the flue gas, for example, a filter oscillation monitor, a carbon particle monitor and the like for use in a concentration measurement field, a cascade impactor, a multi-cyclone and the like for use in a particle size distribution measurement field, and a JIS type dust sampler and the like for use in a dust sampling field for chemical analyses, wherein a remarkable error arises due to non-uniform sampling in case where the flue gas varies, and the measurement is invalidated or to be conducted again.
Accordingly, an object of the present invention is to provide a dust sampling device which is capable of instantaneously changing an opening diameter of a nozzle in response to the change in the flow velocity of the flue gas and to maintain an sokinetic sampling while keeping the sampling flow rate in a measuring device constant as the sampling nozzle is inserted into a chimney or a duct. Namely, in case where the flow rate v of the flue gas is changed, the diameter d of the nozzle inlet is changed so that the relationship, d2v=constant, is kept.
The dust sampling device according to the present invention includes the sampling nozzle with a variable nozzle-inlet diameter. The nozzle is connected to a dust measuring device and a sampling device, and is inserted into an interior of a flue gas passage. The opening diameter of the nozzle is variable so as to keep the sampling flow rate determined by the dust sample measuring device constant, and to have the same sampling velocity as the flow velocity of the flue gas. The flue gas sampled through the nozzle is introduced into the dust measuring device.